I started to write a comment when I noticed that Grzybek had expressed the same opinion as I was going to. I also think it's the way "leave feedback" is tied to the lesson confirmation. I know that you can leave it blank but if you do that then it's too often interpreted as "something was wrong with the lesson but I don't want to say it", when in reality it's just "it was a good lesson, I enjoyed it, but now I have other things to do and I don't want to compose something more original than 'Good lesson!'" which seems repetitive and lame".And if you've got a good teacher then you're getting continual feedback in the lesson anyway, so what's the point?
Students who are less confident in their general language skills may well find themselves intimidated by the need to compose some original thoughts so when they reach the "Hey, leave some feedback!" dialog I will bet you that some of them just cancel out, knowing that eventually the lesson will be auto-accepted anyway but not fully appreciating the impact on the teacher's cashflow.
Italki needs to find a better way of supplying feedback, but that's an observation, not a criticism because I can't think of one either. But it wouldn't surprise me at all if this problem were to diminish if the feedback box was removed, or at least separated to another part of the lesson page.